a) Field of the Invention
The present invention relates to a zoom lens system, and more specifically a zoom lens system permitting photographing at definite magnification levels to provide images at definite magnification levels despite variation of distances to photographed objects.
b) Description of the Prior Art
For photographing objects which are coming nearer or going farther with cinematography cameras, video cameras and so on, it has conventionally been necessary to perform zooming and focusing simultaneously and continuously for photographing images of the objects while keeping constant image sizes on the imaging planes. It is difficult, even with an automatic focusing camera, to perform the operations smoothly for such photographing at a constant magnification level. Especially, highly skilled techniques are required for such photographing with the TV cameras for professional and semi-professional photographers or lowly priced video cameras equipped with no automatic focusing mechanisms.
Further, photographing must be performed at a definite magnification level when academic documents are to be prepared by macro photographing with a still camera. When a macro lens system having a single focal length is to be used for photographing for this purpose, outdoor photographing is difficult and indoor photographing is inconvenient since it is necessary to keep a constant distance from an object to an image thereof. If a zoom lens system is used in this case, it is possible to photograph an object at a constant size despite variation of distance from the object to an image thereof, however inconvenience is caused since focusing and zooming must be performed simultaneously. Furthermore, focal length of the ordinary zoom lens system may be varied when it is focused on an object located at a different distance and complicated operations are required for actual photographing at a definite magnification level.
Moreover, when lens systems are used as eyes of a robot, it is necessary to control the robot on the basis of accurate comprehension of photographing magnification. In case of the ordinary zoom lens systems used as the eyes of a robot, it is necessary to read focused and zoomed conditions on an encoder and determine magnification levels through computations for controlling the robot. That is to say, the robot must be controlled while carrying out computations on the basis of two conditions of focusing and zooming.
A method to solve the above-described problem is disclosed by Japanese Examined Published Patent Application No. 10050/61. This method comprises determination of compensator movement relative to variator as a predetermined movement different from the movement for zooming and photographing at a definite magnification level while performing focusing on an object to be photographed regardless of distance to the object to be photographed.
However, this method requires a cam (hereinafter referred to as "cam for photographing at a definite magnification level") which performs compensator movement different from the ordinary zooming movement, and shape of the cam is determined for a specific magnification level. The shape of the cam must be changed for a different magnification level. In order to make it possible to vary magnification level continuously for photographing at definite magnification levels, it is therefore necessary to change shape of the cam continuously. For making it possible to vary photographing magnification level continuously and mechanically by the above-described method, it is therefore necessary to adopt a complicated mechanism which will be highly priced even with the current electronic control technology.
In addition, it is possible to perform photographing at a nearly definite magnification levels with a relatively lowly priced lens system by using a single cam for photographing at a definite magnification level and performing photographing utilizing depth of field. However, photographing at definite magnification levels will be limited within a narrow range and magnification range will be very narrow especially for macro photographing.